Hemophilia is a hemorrhagic disease caused by congenital defect or dysfunction of coagulation factor VIII (FVIII) or coagulation factor IX (FIX). Hemophilia is called hemophilia A when it is caused by FVIII and called hemophilia B when it is caused by FIX. In hemophilic patients, bleeding symptoms are found in deep tissues such as intraarticular tissues or intramuscular tissues, and intracranial hemorrhage also occurs in severe cases.
The severity of hemophilia is classified based on FVIII activity or FIX activity in the blood. Specifically, patients with an activity of less than 1% are classified as severe, patients with an activity of 1% or more and less than 5% are classified as moderate, and patients with an activity of 5% or more and less than 40% are classified as mild, based on the FVIII activity or FIX activity of a healthy subject as 100%. Patients with severe hemophilia present bleeding symptoms at a significantly higher frequency compared to moderate and mild patients. However, replacement therapy with FVIII or FIX can dramatically reduce the frequency of bleeding by maintaining FVIII activity or FIX activity in the patient's blood at 1% or more.
For replacement therapy, a coagulation factor preparation purified from plasma or prepared by genetic recombination technology is mainly used. In recent years, a bispecific antibody having a FVIII-substituting activity has been developed. This bispecific antibody substitutes for the function as a cofactor of activated coagulation factor VIII (FVIIIa). That is, this bispecific antibody can promote the activation of FX by FIXa by binding to both activated coagulation factor IX (FIXa) and coagulation factor X (FX). This promotes blood coagulation. Meanwhile, the process of blood coagulation promoted by the bispecific antibody does not require activation from FVIII to FVIIIa unlike the normal process by FVIII.
In replacement therapy using a coagulation factor preparation such as FVIII, the efficacy of the administered coagulation factor is monitored by activated partial thromboplastin time (APTT) measurement, coagulation waveform analysis, and the like. However, as described above, since blood coagulation by a bispecific antibody is different from normal blood coagulation by FVIII, it has been difficult to acquire data reflecting the actual efficacy of a bispecific antibody by the existing method for monitoring the efficacy of a coagulation factor preparation.
Under such circumstances, the present inventors have found so far that the efficacy of a substance having a FVIII-substituting activity can be evaluated with appropriate sensitivity using the thrombin generation amount in a blood specimen as an index (see Patent Literature 1). However, this method requires a special measuring equipment, so it has not been widely spread as a clinical test. In addition, the present inventors have found that the factor VIII-substituting activity of the bispecific antibody can be measured by coagulation waveform analysis using a commercially available APTT measuring reagent (see Non Patent Literature 1). However, with this method, the efficacy of the bispecific antibody could not be evaluated with appropriate sensitivity.